This is meant directly for CA, but do write if you agree.
I think that Lü Bu's greatest warrios should be completed if you hired a character from the list. The driving behind this is to achieve all of these. You want Lü Bu to become as strong as possible, but at the same time, you want all of the greatest characters for yourself. So if you destroy a faction, but don't meet the legendary generals in battle, they might become able in the charactor pool, where it is very bitter if you don't hire them.
You basically put people in a position where they can't have all they want, and that annoys them. Secondly, you could easily justify this. If a character is hired by Lü Bu, they bend their knee to him, admitting that he is their superior. If he had to beat them in battle, cou could even make an event where they fight a 1v1.
Love the Sun Ce though. Nothing stopping him reaching his personal goals.